1) Figure out what the maximum voltage is that you are not willing to go over.
2) Figure out what the maximum temperature is that you are not willing to go over. (Usually goes hand and hand with voltage)
These are dependent on your CPU, cooling, PSU and seeing what others have discovered as a bad voltage for your CPU. You can find rules of thumb on this from people who have experimented with how far they could push their CPU before the silicon degrades and it does not over clock any more or just plain does not work.
If you are not comfortable with the idea that you might damage your CPU or at least shorten its life expectancy then I would suggest you use the over clocking Genie that comes with most BIOS.
If you know this and what your limits are then you simply raise the multiplier until you system does not boot then raise your voltage until it does boot. Then you run some kind of CPU intensive program like Prime 95 along with a temperature sensor to see what the temps. If the system crashes then raise the voltage until it either does not crash or you meet you maximum that you are not wiling to cross or if you reach the maximum temperature that you are not willing to cross.
There is usually a sweet sport were the ratio of clock speed increases to voltage increases go over the edge were you need an extreme amount of volts to go a tiny amount of clock increase. I usually try to keep it just under the sweet spot.
Good luck.